The Na+ /Ca2+ antiporter slr0681 affects carotenoid production in Synechocystis sp. PCC 6803 under high-light stress.

BACKGROUND: Carotenoids play key roles in photosynthesis and are widely used in foods as natural pigments, antioxidants, and health-promoting compounds. Enhancing carotenoid production in microalgae via biotechnology has become an important area of research. RESULTS: We knocked out the Na+ /Ca2+ antiporter gene slr0681 in Synechocystis sp. PCC 6803 via homologous recombination and evaluated the effects on carotenoid production under normal (NL) and high-light (HL) conditions. On day 7 of NL treatment in calcium ion (Ca2+ )-free medium, the cell density of Δslr0681 decreased by 29% compared to the wild type (WT). After 8 days of HL treatment, the total carotenoid contents decreased by 35% in Δslr0681, and the contents of individual carotenoids were altered: myxoxanthophyll, echinenone, and ß-carotene contents increased by 10%, 50%, and 40%, respectively, while zeaxanthin contents decreased by ~40% in Δslr0681 versus the WT. The expression patterns of carotenoid metabolic pathway genes also differed: ipi expression increased by 1.2- to 8.5-fold, whereas crtO and crtR expression decreased by ~90% and 60%, respectively, in ∆slr0681 versus the WT. In addition, in ∆slr0681, the expression level of psaB (encoding a photosystem I structural protein) doubled, whereas the expression levels of the photosystem II genes psbA2 and psbD decreased by ~53% and 84%, respectively, compared to the WT. CONCLUSION: These findings suggest that slr0681 plays important roles in regulating carotenoid biosynthesis and structuring of the photosystems in Synechocystis sp. This study provides a theoretical basis for the genetic engineering of microalgae photosystems to increase their economic benefits and lays the foundation for developing microalgae germplasm resources with high carotenoid contents. © 2023 Society of Chemical Industry.

Light limitation inducing overcompensatory growth of cyanobacteria and function of serine/threonine kinase (STK) genes involved.

The rapid overcompensatory growth that appears when cyanobacteria are supplied with adequate resources after a period of resource deprivation might contribute to the occurrence of cyanobacterial blooms. We investigated the changing characteristics of overcompensatory growth and serine/threonine kinase (STK) genes expression of cyanobacterium Microcystis aeruginosa in response to light limitation. The results showed M. aeruginosa exhibited overcompensatory growth for 2 days after light recovery, during which the increase in growth was inversely related to light intensity. Expression of STK genes, such as spkD, was upregulated significantly at 0.5-4 h after light recovery (P < 0.05). To investigate the function of STK genes in the overcompensatory growth, M. aeruginosa spkD was heterologously expressed in Synechocystis. Transgenic Synechocystis exhibited greater and longer overcompensatory growth than wild-type Synechocystis after light recovery. Relative expression levels of STK genes in transgenic Synechocystis were significantly higher than those in wild-type Synechocystis at 24 h of light recovery (P < 0.05). Heterologous expression of Microcystis spkD might stimulate overcompensatory growth of Synechocystis by affecting its STK gene expression.

The Effects of Different Degrees of Procyanidin Polymerization on the Nutrient Absorption and Digestive Enzyme Activity in Mice.

Proanthocyanidins, including polymers with both low and high degrees of polymerization, are the focus of intensive research worldwide due to their high antioxidant activity, medicinal applications, and pharmacological properties. However, the nutritional value of these compounds is limited because they readily form complexes with proteins, polysaccharides, and metal ions when consumed. In this study, we examined the effects of proanthocyanidins with different degrees of polymerization on white mice. Twenty-four male white mice were randomly divided into three groups of eight mice each and fed proanthocyanidins with a low degree of polymerization or a high degree of polymerization or a distilled water control via oral gavage over a 56-day period. We examined the effects of these proanthocyanidins on digestive enzyme activity and nutrient absorption. Compared to the control group, the group fed high-polymer proanthocyanidins exhibited a significant reduction in net body mass, total food intake, food utility rate, amylase activity, protease activity, and major nutrient digestibility (p < 0.05), while the group fed low-polymerization proanthocyanidins only exhibited significant reductions in total food intake, α-amylase activity, and apparent digestibility of calcium and zinc (p < 0.05). Therefore, proanthocyanidins with a high degree of polymerization had a greater effect on digestive enzyme activity and nutrient absorption than did those with a low degree of polymerization. This study lays the foundation for elucidating the relationship between procyanidin polymerization and nutrient uptake, with the aim of reducing or eliminating the antinutritional effects of polyphenols.

Serine/threonine Kinases Play Important Roles in Regulating Polyunsaturated Fatty Acid Biosynthesis in Synechocystis sp. PCC6803.

Serine/threonine kinases (STKs) play important roles in prokaryotic cellular functions such as growth, differentiation, and secondary metabolism. When the external environment changes, prokaryotes rely on signal transduction systems, including STKs that quickly sense these changes and alter gene expression to induce the appropriate metabolic changes. In this study, we examined the roles of the STK genes spkD and spkG in fatty acid biosynthesis in the unicellular cyanobacterium Synechocystis sp. PCC6803, using targeted gene knockout. The linoleic acid (C18: 2), γ-linolenic acid (C18: 3n6), α-linolenic acid (C18: 3n3), and stearidonic acid (C18: 4) levels were significantly lower in spkD and spkG gene knockout mutants than in the wild type at a culture temperature of 30°C and a light intensity of 40 µmol⋅m-2⋅s-1. The expression levels of fatty acid desaturases and STK genes differed between the spkD and spkG gene knockout mutants. These observations suggest that spkD and spkG may directly or indirectly affect the fatty acid composition in Synechocystis sp. PCC6803 by regulating the expression of fatty acid desaturases genes. Therefore, the STK genes spkD and spkG play important roles in polyunsaturated fatty acid biosynthesis in Synechocystis sp. PCC6803. These findings could facilitate the development of cyanobacteria germplasm resources that yield high levels of fatty acids. In addition, they provide a theoretical basis for the genetic engineering of cyanobacteria with improved yields of secondary metabolites and increased economic benefits.